High temperature oxidation protection for composites

ABSTRACT

The present disclosure provides a method for coating a composite structure, comprising forming a first slurry by combining a first pre-slurry composition comprising a first phosphate glass composition, with a primary flow modifier and a first carrier fluid, wherein the primary flow modifier comprises at least one of cellulose or calcium silicate; applying the first slurry on a surface of the composite structure to form a base layer; and heating the composite structure to a temperature sufficient to adhere the base layer to the composite structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of, and claims priority to and thebenefit of, U.S. Ser. No. 16/668,852, filed Oct. 30, 2019 and entitled“HIGH TEMPERATURE OXIDATION PROTECTION FOR COMPOSITES.” The '852application is Divisional of, and claims priority to and the benefit of,U.S. Ser. No. 15/380,442, filed Dec. 15, 2016 and entitled “HIGHTEMPERATURE OXIDATION PROTECTION FOR COMPOSITES,” now U.S. Pat. No.10,526,253 issued on Jan. 7, 2020. All of which are hereby incorporatedby reference herein in their entirety for all purposes.

FIELD

The present disclosure relates generally to carbon-carbon compositesand, more specifically, to oxidation protection systems forcarbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typicallydesigned to minimize loss of carbon material due to oxidation atoperating conditions, which include temperatures as high as 900° C.(1652° F.). Phosphate-based oxidation protection systems may reduceinfiltration of oxygen and oxidation catalysts into the compositestructure. However, despite the use of such oxidation protectionsystems, significant oxidation of the carbon-carbon composites may stilloccur during operation of components such as, for example, aircraftbraking systems.

SUMMARY

A method for coating a composite structure (e.g., a carbon-carboncomposite structure) is provided comprising forming a first slurry bycombining a first pre-slurry composition comprising a first phosphateglass composition, with a primary flow modifier and a first carrierfluid, wherein the primary flow modifier comprises at least one ofcellulose or calcium silicate; applying the first slurry on a surface ofthe composite structure to form a base layer; and/or heating thecomposite structure to a temperature sufficient to adhere the base layerto the composite structure. Forming the first slurry may comprise mixingthe first pre-slurry composition and the first carrier fluid prior tomixing the primary flow modifier into the first slurry.

In various embodiments, the first pre-slurry composition may furthercomprise a secondary flow modifier that comprises at least one ofpoly(vinyl alcohol) or a salt of poly(acrylic acid). In variousembodiments, the first pre-slurry composition may comprise an acidaluminum phosphate, wherein a molar ratio of aluminum to phosphoric acidis between 1 to 2 and 1 to 3.

In various embodiments, the method may further comprise forming a secondslurry by combining a second pre-slurry composition with a secondcarrier fluid, wherein the second pre-slurry composition comprises asecond phosphate glass composition; applying the second slurry to thebase layer; and heating the composite structure to a second temperaturesufficient to form a sealing layer on the base layer.

In various embodiments, the method may further comprise applying atleast one of a pretreating composition or a barrier coating to thecomposite structure prior to applying the first slurry to the compositestructure. In various embodiments, the pretreating composition maycomprise at least one of a phosphoric acid or an acid phosphate salt, analuminum salt, or an additional salt, and the composite structure may beporous and the pretreating composition may penetrate a pore of thecomposite structure. In various embodiments, applying a pretreatingcomposition may comprise applying a first pretreating composition to anouter surface of the composite structure, the first pretreatingcomposition comprising aluminum oxide and water; heating the pretreatingcomposition; and applying a second pretreating composition comprising atleast one of a phosphoric acid or an acid phosphate salt and an aluminumsalt on the first pretreating composition, wherein the compositestructure is porous and the second pretreating composition penetrates apore of the composite structure. In various embodiments, the barriercoating may comprise at least one of a carbide, a nitride, a boronnitride, a silicon carbide, a titanium carbide, a boron carbide, asilicon oxycarbide, a molybdenum disulfide, a tungsten disulfide, or asilicon nitride.

In various embodiments, the first phosphate glass composition and/or thesecond phosphate glass composition is represented by the formulaa(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, andmixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium,manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead,zirconium, lanthanum, cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium,calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y ₁ +y ₂ +z)=1; and

x<(y ₁ +y ₂).

In various embodiments, a slurry for coating a composite structure maycomprise a carrier fluid; a primary flow modifier comprising at leastone of cellulose or calcium silicate; and a first pre-slurry compositioncomprising a first phosphate glass composition. In various embodiments,the first pre-slurry composition may further comprise a secondary flowmodifier that is at least one of poly(vinyl alcohol) or a salt ofpoly(acrylic acid).

In accordance with various embodiments, an article may comprise acarbon-carbon composite structure; and an oxidation protection systemincluding a base layer disposed on an outer surface of the carbon-carbonstructure, wherein the base layer may comprise a first pre-slurrycomposition having a first phosphate glass composition, and a primaryflow modifier, wherein the primary flow modifier is at least one ofcellulose or calcium silicate. In various embodiments, the firstpre-slurry composition of the base layer may comprise a secondary flowmodifier that is at least one of poly(vinyl alcohol) or a salt ofpoly(acrylic acid). In various embodiments, the first pre-slurrycomposition of the base layer may comprise an acid aluminum phosphatewherein a molar ratio of aluminum to phosphoric acid is between 1 to 2and 1 to 3. In various embodiments, the article may further comprise asealing layer disposed on an outer surface of the base layer, whereinthe sealing layer may comprise a second pre-slurry compositioncomprising a second phosphate glass composition. In various embodiments,the second pre-slurry composition may comprise acid aluminum phosphateand wherein the second phosphate glass composition is substantially freeof boron nitride.

In various embodiments, the first phosphate glass composition or thesecond phosphate glass composition may be represented by the formulaa(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, andmixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium,manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead,zirconium, lanthanum, cerium, praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium,calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y ₁ +y ₂ +z)=1; and

x<(y ₁ +y ₂).

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present disclosure, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements.

FIG. 1A illustrates a cross sectional view of an aircraft wheel brakingassembly, in accordance with various embodiments;

FIG. 1B illustrates a partial side view of an aircraft wheel brakingassembly, in accordance with various embodiments; and

FIGS. 2A, 2B, and 2C illustrate a method for coating a compositestructure, in accordance with various embodiments; and

FIG. 3 illustrates experimental data obtained from testing variousoxidation protection systems on composite structures, in accordance withvarious embodiments.

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to theaccompanying drawings, which show embodiments by way of illustration.While these embodiments are described in sufficient detail to enablethose skilled in the art to practice the disclosure, it should beunderstood that other embodiments may be realized and that logical andmechanical changes may be made without departing from the spirit andscope of the disclosure. Thus, the detailed description herein ispresented for purposes of illustration only and not for limitation. Forexample, any reference to singular includes plural embodiments, and anyreference to more than one component or step may include a singularembodiment or step. Also, any reference to attached, fixed, connected orthe like may include permanent, removable, temporary, partial, fulland/or any other possible attachment option.

With initial reference to FIGS. 1A and 1B, aircraft wheel brakingassembly 10 such as may be found on an aircraft, in accordance withvarious embodiments is illustrated. Aircraft wheel braking assembly may,for example, comprise a bogie axle 12, a wheel 14 including a hub 16 anda wheel well 18, a web 20, a torque take-out assembly 22, one or moretorque bars 24, a wheel rotational axis 26, a wheel well recess 28, anactuator 30, multiple brake rotors 32, multiple brake stators 34, apressure plate 36, an end plate 38, a heat shield 40, multiple heatshield sections 42, multiple heat shield carriers 44, an air gap 46,multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52,multiple heat shield fasteners 53, multiple rotor lugs 54, and multiplestator slots 56. FIG. 1B illustrates a portion of aircraft wheel brakingassembly 10 as viewed into wheel well 18 and wheel well recess 28.

In various embodiments, the various components of aircraft wheel brakingassembly 10 may be subjected to the application of compositions andmethods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed inwheel well recess 28 of wheel well 18. Rotors 32 are secured to torquebars 24 for rotation with wheel 14, while stators 34 are engaged withtorque take-out assembly 22. At least one actuator 30 is operable tocompress interleaved rotors 32 and stators 34 for stopping the aircraft.In this example, actuator 30 is shown as a hydraulically actuatedpiston, but many types of actuators are suitable, such as anelectromechanical actuator. Pressure plate 36 and end plate 38 aredisposed at opposite ends of the interleaved rotors 32 and stators 34.Rotors 32 and stators 34 can comprise any material suitable for frictiondisks, including ceramics or carbon materials, such as a carbon/carboncomposite.

Through compression of interleaved rotors 32 and stators 34 betweenpressure plates 36 and end plate 38, the resulting frictional contactslows rotation of wheel 14. Torque take-out assembly 22 is secured to astationary portion of the landing gear truck such as a bogie beam orother landing gear strut, such that torque take-out assembly 22 andstators 34 are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as compositestructures, composite substrates, and carbon-carbon compositestructures, interchangeably) in the friction disks may operate as a heatsink to absorb large amounts of kinetic energy converted to heat duringslowing of the aircraft. Heat shield 40 may reflect thermal energy awayfrom wheel well 18 and back toward rotors 32 and stators 34. Withreference to FIG. 1A, a portion of wheel well 18 and torque bar 24 isremoved to better illustrate heat shield 40 and heat shield segments 42.With reference to FIG. 1B, heat shield 40 is attached to wheel 14 and isconcentric with wheel well 18. Individual heat shield sections 42 may besecured in place between wheel well 18 and rotors 32 by respective heatshield carriers 44 fixed to wheel well 18. Air gap 46 is definedannularly between heat shield segments 42 and wheel well 18.

Torque bars 24 and heat shield carriers 44 can be secured to wheel 14using bolts or other fasteners. Torque bar bolts 48 can extend through ahole formed in a flange or other mounting surface on wheel 14. Eachtorque bar 24 can optionally include at least one torque bar pin 50 atan end opposite torque bar bolts 48, such that torque bar pin 50 can bereceived through wheel web hole 52 in web 20. Heat shield sections 42and respective heat shield carriers 44 can then be fastened to wheelwell 18 by heat shield fasteners 53.

Under the operating conditions (e.g., high temperature) of aircraftwheel braking assembly 10, carbon-carbon composites may be prone tomaterial loss from oxidation of the carbon. For example, variouscarbon-carbon composite components of aircraft wheel braking assembly 10may experience both catalytic oxidation and inherent thermal oxidationcaused by heating the composite during operation. In variousembodiments, composite rotors 32 and stators 34 may be heated tosufficiently high temperatures that may oxidize the carbon surfacesexposed to air. At elevated temperatures, infiltration of air andcontaminants may cause internal oxidation and weakening, especially inand around brake rotor lugs 54 or stator slots 56 securing the frictiondisks to the respective torque bar 24 and torque take-out assembly 22.Because carbon-carbon composite components of aircraft wheel brakingassembly 10 may retain heat for a substantial time period after slowingthe aircraft, oxygen from the ambient atmosphere may react with thecarbon matrix and/or carbon fibers to accelerate material loss. Further,damage to brake components may be caused by the oxidation enlargement ofcracks around fibers or enlargement of cracks in a reaction-formedporous barrier coating (e.g., a silicon-based barrier coating) appliedto the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carboncomposite brake components include potassium (K) and sodium (Na). Thesealkali contaminants may come into contact with aircraft brakes as partof cleaning or de-icing materials. Other sources include salt depositsleft from seawater or sea spray. These and other contaminants (e.g. Ca,Fe, etc.) can penetrate and leave deposits in pores of carbon-carboncomposite aircraft brakes, including the substrate and anyreaction-formed porous barrier coating. When such contamination occurs,the rate of carbon loss by oxidation can be increased by one to twoorders of magnitude.

In various embodiments, components of aircraft wheel braking assembly 10may reach operating temperatures in the range from about 100° C. (212°F.) up to about 900° C. (1652° F.). However, it will be recognized thatthe oxidation protection compositions, systems, and methods of thepresent disclosure may be readily adapted to many parts in this andother braking assemblies, as well as to other carbon-carbon compositestructures susceptible to oxidation losses from infiltration ofatmospheric oxygen and/or catalytic contaminants. An oxidationprotection system applied to a composite structure may prevent or reduceoxidation of the composite structure. In various embodiments, anoxidation protection system may comprise a base layer resulting from afirst slurry, a sealing layer resulting from a second slurry, and/orlayers and compositions.

In various embodiments, a method for limiting an oxidation reaction in acomposite structure may comprise forming a first slurry by combining afirst pre-slurry composition comprising a first phosphate glasscomposition in the form of a glass frit, powder, or other suitablepulverized form, with a first carrier fluid (such as, for example,water), applying the first slurry to a composite structure, and heatingthe composite structure to a temperature sufficient to dry the carrierfluid and form an oxidation protection coating on the compositestructure, which in various embodiments may be referred to a base layer.The first pre-slurry composition of the first slurry may compriseadditives, such as, for example, ammonium hydroxide, ammonium dihydrogenphosphate, nanoplatelets (such as graphene-based nanoplatelets), amongothers, to improve hydrolytic stability and/or to increase the compositestructure's resistance to oxidation, thereby tending to reduce mass lossof the composite structure. In various embodiments, the first slurry maycomprise a primary flow modifier. The primary flow modifier may be mixedwith the first pre-slurry composition after the pre-slurry compositionis combined with the carrier fluid.

In various embodiments, a slurry comprising acid aluminum phosphateshaving an aluminum (Al) to phosphoric acid (H₃PO₄) molar ratio of 1 to 3or less, such as an Al:H₃PO₄ molar ratio of between 1 to 2 and 1 to 3,tends to provide increased hydrolytic stability without substantiallyincreasing composite structure mass loss. In various embodiments, aslurry comprising acid aluminum phosphates having an Al:H₃PO₄ molarratio between 1:2 to 1:3 produces an increase in hydrolytic protectionand an unexpected reduction in composite structure mass loss.

With initial reference to FIG. 2A, a method 200 for coating a compositestructure in accordance with various embodiments is illustrated. Method200 may, for example, comprise applying an oxidation protection systemto non-wearing surfaces of carbon-carbon composite brake components. Invarious embodiments, method 200 may be used on the back face of pressureplate 36 and/or end plate 38, an inner diameter (ID) surface of stators34 including slots 56, as well as outer diameter (OD) surfaces of rotors32 including lugs 54. The oxidation inhibiting composition of method 200may be applied to preselected regions of a carbon-carbon compositestructure that may be otherwise susceptible to oxidation. For example,aircraft brake disks may have the oxidation inhibiting compositionapplied on or proximate stator slots 56 and/or rotor lugs 54.

In various embodiments, method 200 may comprise forming a first slurry(step 210) by combining a first pre-slurry composition, comprising afirst phosphate glass composition in the form of a glass frit, powder,or other suitable pulverized and/or ground form, with a first carrierfluid (such as, for example, water). In various embodiments, the firstslurry may comprise an acid aluminum phosphate wherein the molar ratioof Al:H₃PO₄ may be between 1:2 to 1:3, between 1:2.2 to 1:3, between1:2.5 to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3. The firstpre-slurry composition of the first slurry may further comprise a boronnitride additive. For example, a boron nitride (such as hexagonal boronnitride) may be added such that the resulting first pre-slurrycomposition comprises between about 5 weight percent and about 30 weightpercent of boron nitride. Further, the pre-slurry composition maycomprise between about 15 weight percent and 25 weight percent of boronnitride. As used in this context only, the term “about” means plus orminus 2 weight percent. Boron nitride may be prepared for addition tothe first pre-slurry composition by, for example, ultrasonicallyexfoliating boron nitride in dimethylformamide (DMF), a solution of DMFand water, or 2-propanol solution. In various embodiments, the boronnitride additive may comprise a boron nitride that has been prepared foraddition to the first phosphate glass composition by crushing or milling(e.g., ball milling) the boron nitride. The resulting boron nitride maybe combined with the first phosphate glass composition.

In various embodiments, the first slurry may comprise a primary flowmodifier, such as hydrophilic bentonite, sodium metasilicate, poly(vinylalcohol), a salt of poly(acrylic acid), poly(vinyl acetate), calciumsilicate, cellulose, kaolin, and/or mullite. In various embodiments, theprimary flow modifier may be added to the first slurry during the mixingof the components of the first pre-slurry composition, or after all thecomponents of the first pre-slurry composition are combined. In variousembodiments, the primary flow modifier may be added to the first slurryafter all the components of the first pre-slurry composition arecombined with the carrier fluid. The presence of the primary flowmodifier in the first slurry may enhance the bonding or interaction ofthe molecules of the first slurry to form a substantially even and/orcontinuous distribution of the first slurry on a composite structure.For example, cellulose comprised in the first slurry surprisingly bondsand/or interacts well with boron nitride, causing the dispersion of thefirst slurry and the resulting base layer to be more continuous acrossthe composite structure, such that there are few or no gaps in the firstslurry and the resulting base layer, as opposed to a first slurrywithout cellulose. Additionally, the primary flow modifier may alsoprevent or reduce sagging of the first slurry after application to thecomposite structure.

In various embodiments, the first slurry may comprise about 2% to about12% by weight primary flow modifier, about 4% to about 10% by weightprimary flow modifier, or about 5% to about 7% by weight primary flowmodifier. The term “about” as used in this context only means plus orminus 1% by weight. In various embodiments, cellulose particles used inthe first slurry may have any suitable particle size. In variousembodiments, a particle size of 10 micrometers (0.00004 inch) to 50micrometers (0.0020 inch), 15 micrometers (0.0006 inch) to 40micrometers (0.0016 inch), 15 micrometers (0.0006 inch) to 30micrometers (0.0012 inch), or 18 micrometers (0.0007 inch) to 22micrometers (0.0009 inch).

The first phosphate glass composition may comprise one or more alkalimetal glass modifiers, one or more glass network modifiers and/or one ormore additional glass formers. In various embodiments, boron oxide or aprecursor may optionally be combined with the P₂O₅ mixture to form aborophosphate glass, which has improved self-healing properties at theoperating temperatures typically seen in aircraft braking assemblies. Invarious embodiments, the phosphate glass and/or borophosphate glass maybe characterized by the absence of an oxide of silicon. Further, theratio of P₂O₅ to metal oxide in the fused glass may be in the range fromabout 0.25 to about 5 by weight.

Potential alkali metal glass modifiers may be selected from oxides oflithium, sodium, potassium, rubidium, cesium, and mixtures thereof. Invarious embodiments, the glass modifier may be an oxide of lithium,sodium, potassium, or mixtures thereof. These or other glass modifiersmay function as fluxing agents. Additional glass formers can includeoxides of boron, silicon, sulfur, germanium, arsenic, antimony, andmixtures thereof.

Suitable glass network modifiers include oxides of vanadium, aluminum,tin, titanium, chromium, manganese, iron, cobalt, nickel, copper,mercury, zinc, thulium, lead, zirconium, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium,uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin,bismuth, cadmium, and mixtures thereof.

The first phosphate glass composition may be prepared by combining theabove ingredients and heating them to a fusion temperature. In variousembodiments, depending on the particular combination of elements, thefusion temperature may be in the range from about 700° C. (1292° F.) toabout 1500° C. (2732° F.). The resultant melt may then be cooled andpulverized and/or ground to form a glass frit or powder. In variousembodiments, the first phosphate glass composition may be annealed to arigid, friable state prior to being pulverized. Glass transitiontemperature (T_(g)), glass softening temperature (T_(s)) and glassmelting temperature (T_(m)) may be increased by increasing refinementtime and/or temperature. Before fusion, the first phosphate glasscomposition comprises from about 20 mol % to about 80 mol % of P₂O₅. Invarious embodiments, the first phosphate glass composition comprisesfrom about 30 mol % to about 70 mol % P₂O₅, or precursor thereof. Invarious embodiments, the first phosphate glass composition comprisesfrom about 40 to about 60 mol % of P₂O₅.

The first phosphate glass composition may comprise from about 5 mol % toabout 50 mol % of the alkali metal oxide. In various embodiments, thefirst phosphate glass composition comprises from about 10 mol % to about40 mol % of the alkali metal oxide. Further, the first phosphate glasscomposition comprises from about 15 to about 30 mol % of the alkalimetal oxide or one or more precursors thereof. In various embodiments,the first phosphate glass composition may comprise from about 0.5 mol %to about 50 mol % of one or more of the above-indicated glass formers.The first phosphate glass composition may comprise about 5 to about 20mol % of one or more of the above-indicated glass formers. As usedherein, mol % is defined as the number of moles of a constituent per thetotal moles of the solution.

In various embodiments, the first phosphate glass composition maycomprise from about 0.5 mol % to about 40 mol % of one or more of theabove-indicated glass network modifiers. The first phosphate glasscomposition may comprise from about 2.0 mol % to about 25 mol % of oneor more of the above-indicated glass network modifiers.

In various embodiments, the first phosphate glass composition may berepresented by the formula:

a(A′ ₂ O)_(x)(P ₂ O ₅)_(y1) b(G _(f) O)_(y2) c(A″O)_(z)  [1]

In Formula 1, A′ is selected from: lithium, sodium, potassium, rubidium,cesium, and mixtures thereof; G_(f) is selected from: boron, silicon,sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ isselected from: vanadium, aluminum, tin, titanium, chromium, manganese,iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium,lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,actinium, thorium, uranium, yttrium, gallium, magnesium, calcium,strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is anumber in the range from 1 to about 5; b is a number in the range from 0to about 10; c is a number in the range from 0 to about 30; x is anumber in the range from about 0.050 to about 0.500; y₁ is a number inthe range from about 0.100 to about 0.950; y₂ is a number in the rangefrom 0 to about 0.20; and z is a number in the range from about 0.01 toabout 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂). The first phosphate glasscomposition may be formulated to balance the reactivity, durability andflow of the resulting glass barrier layer for optimal performance.

In various embodiments, first phosphate glass composition in glass fritform may be combined with additional components to form the firstpre-slurry composition. For example, crushed first phosphate glasscomposition in glass frit form may be combined with ammonium hydroxide,ammonium dihydrogen phosphate, nanoplatelets (such as graphene-basednanoplatelets), among other materials and/or substances. For example,graphene nanoplatelets could be added to the first phosphate glasscomposition in glass frit form. In various embodiments, the additionalcomponents of the first pre-slurry composition may be combined andpreprocessed before combining them with the first phosphate glasscomposition in glass frit form. Other suitable additional componentsinclude, for example, surfactants such as, for example, an ethoxylatedlow-foam wetting agent, and/or a secondary flow modifier, such as, forexample, poly(vinyl alcohol), polyacrylate salt(s) or poly(acrylicacid), and/or similar polymers. In various embodiments, the addition ofa secondary flow modifier may decrease the viscosity of the first slurrysuch that the first slurry may be more easily applied (such as in step220, described herein) via a spraying method. In various embodiments,the first slurry may comprise between 0.1% and 2.0% by weight secondaryflow modifier, between 0.3% and 1.5% by weight secondary flow modifier,or between 0.5% and 1.2% by weight secondary flow modifier.

In various embodiments, in which the secondary flow modifier is a saltof poly(acrylic acid) (e.g., sodium salt of poly(acrylic acid)), theparticle size used in the first slurry may be between 1250 and 7500grams per mole (g/mol), 1500 and 6000 g/mol, 1700 and 2500 g/mol, or4500 and 5500 g/mol.

In various embodiments, other suitable additional components may includeadditives to enhance impact resistance and/or to toughen the base layer,such as, for example, at least one of whiskers, nanofibers or nanotubesconsisting of nitrides, carbides, carbon, graphite, quartz, silicates,aluminosilicates, phosphates, and the like. In various embodiments,additives to enhance impact resistance and/or to toughen the barriercoating may include silicon carbide whiskers, carbon nanofibers, boronnitride nanotubes and similar materials known to those skilled in theart.

In various embodiments, method 200 further comprises applying the firstslurry to a composite structure (step 220). Applying the first slurrymay comprise, for example, spraying or brushing the first slurry of thefirst phosphate glass composition on to an outer surface of thecomposite structure. Any suitable manner of applying the base layer tothe composite structure is within the scope of the present disclosure.As referenced herein, the composite structure may refer to acarbon-carbon composite structure.

In various embodiments, method 200 may further comprise a step 230 ofheating the composite structure to form a base layer of phosphate glass.The composite structure may be heated (e.g., dried or baked) at atemperature in the range from about 200° C. (292° F.) to about 1000° C.(1832° F.). In various embodiments, the composite structure is heated toa temperature in a range from about 600° C. (1112° F.) to about 1000° C.(1832° F.), or between about 200° C. (292° F.) to about 900° C. (1652°F.), or further, between about 400° C. (752° F.) to about 850° C. (1562°F.). Step 230 may, for example, comprise heating the composite structurefor a period between about 0.5 hour and about 8 hours, wherein the term“about” in this context only means plus or minus 0.25 hours. The baselayer may also be referred to as a coating.

In various embodiments, the composite structure may be heated to afirst, lower temperature (for example, about 30° C. (86° F.) to about400° C. (752° F.)) to bake or dry the base layer at a controlled depth.A second, higher temperature (for example, about 300° C. (572° F.) toabout 1000° C. (1832° F.)) may then be used to form a deposit from thebase layer within the pores of the composite structure. The duration ofeach heating step can be determined as a fraction of the overall heatingtime and can range from about 10% to about 50%, wherein the term “about”in this context only means plus or minus 5%. In various embodiments, theduration of the lower temperature heating step(s) can range from about20% to about 40% of the overall heating time, wherein the term “about”in this context only means plus or minus 5%. The lower temperaturestep(s) may occupy a larger fraction of the overall heating time, forexample, to provide relatively slow heating up to and through the firstlower temperature. The exact heating profile will depend on acombination of the first temperature and desired depth of the dryingportion.

Step 230 may be performed in an inert environment, such as under ablanket of inert gas or less reactive gas (e.g., nitrogen, argon, othernoble gases and the like). For example, a composite structure may bepretreated or warmed prior to application of the base layer to aid inthe penetration of the base layer. Step 230 may be for a period of about2 hours at a temperature of about 600° C. (1112° F.) to about 800° C.(1472° F.), wherein the term “about” in this context only means plus orminus 10° C. The composite structure and base layer may then be dried orbaked in a non-oxidizing, inert or less reactive atmosphere, e.g., noblegasses and/or nitrogen (N₂), to optimize the retention of the firstpre-slurry composition of the base layer in the pores of the compositestructure. This retention may, for example, be improved by heating thecomposite structure to about 200° C. (392° F.) and maintaining thetemperature for about 1 hour before heating the carbon-carbon compositeto a temperature in the range described above. The temperature rise maybe controlled at a rate that removes water without boiling, and providestemperature uniformity throughout the composite structure.

In various embodiments and with reference now to FIG. 2B, method 300,which comprises steps also found in method 200, may further compriseapplying at least one of a pretreating composition or a barrier coating(step 215) prior to applying the first slurry. Step 215 may, forexample, comprise applying a first pretreating composition to an outersurface of a composite structure, such as a component of aircraft wheelbraking assembly 10. In various embodiments, the first pretreatingcomposition comprises an aluminum oxide in water. For example, thealuminum oxide may comprise an additive, such as a nanoparticledispersion of aluminum oxide (for example, NanoBYK-3600®, sold by BYKAdditives & Instruments). The first pretreating composition may furthercomprise a surfactant or a wetting agent. The composite structure may beporous, allowing the pretreating composition to penetrate at least aportion of the pores of the composite structure.

In various embodiments, after applying the first pretreatingcomposition, the component may be heated to remove water and fix thealuminum oxide in place. For example, the component may be heated tobetween about 100° C. (212° F.) and 200° C. (392° F.), and further,between 100° C. (212° F.) and 150° C. (302° F.).

Step 215 may further comprise applying a second pretreating composition.In various embodiments, the second pretreating composition may comprisean acid aluminum phosphate, which may be comprised of at least one ofphosphoric acid or an acid phosphate salt, and an aluminum salt (e.g.,aluminum phosphate, aluminum hydroxide, and/or aluminum oxide). Thesecond pretreating composition may further comprise, for example, asecond metal salt such as a magnesium salt. In various embodiments, thealuminum to phosphorus molar ratio of the acid aluminum phosphate may be1 to 3 or less. Further, the second pretreating composition may alsocomprise a surfactant or a wetting agent. In various embodiments, thesecond pretreating composition is applied to the composite structureatop the first pretreating composition. The composite structure maythen, for example, be heated. In various embodiments, the compositestructure may be heated between about 600° C. (1112° F.) and about 800°C. (1472° F.), and further, between about 650° C. (1202° F.) and 750° C.(1382° F.).

Step 215 may further comprise applying a barrier coating to an outersurface of a composite structure, such as a component of aircraft wheelbraking assembly 10. In various embodiments the barrier coatingcomposition may comprise carbides and/or nitrides, including at leastone of a boron nitride, silicon carbide, titanium carbide, boroncarbide, silicon oxycarbide, and silicon nitride. In variousembodiments, the barrier coating composition may comprise molybdenumdisulfide and/or tungsten disulfide instead of, or in addition to,carbides and/or nitrides. In various embodiments, the barrier coatingmay be formed by treating the composite structure with molten silicon.The molten silicon is reactive and may form a silicon carbide barrier onthe composite structure. Step 215 may comprise, for example, applicationof the barrier coating by spraying, chemical vapor deposition (CVD),molten application, or brushing the barrier coating composition on tothe outer surface of the carbon-carbon composite structure. Any suitablemanner of applying the base layer to composite structure is within thescope of the present disclosure.

In various embodiments and with reference now to FIG. 2C, method 400 mayfurther comprise a step 240, similar to step 210, of forming a secondslurry by combining a second pre-slurry composition, which may comprisea second phosphate glass composition in glass frit or powder form, witha second carrier fluid (such as, for example, water). In variousembodiments, the second slurry may comprise an acid aluminum phosphatewherein the molar ratio of aluminum (Al) to phosphoric acid (H₃PO₄) maybe between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3,between 1:2.7 to 1:3 or between 1:2.9 to 1:3. In various embodiments,the second slurry may comprise a second pre-slurry compositioncomprising acid aluminum phosphate and orthophosphoric acid with analuminum to phosphate molar ratio of 1:2 to 1:5, and may besubstantially phosphate glass free. As used herein “substantially free”means comprising less than 0.01% by weight of a substance. Further, step240 may comprise spraying or brushing the second slurry of the secondphosphate glass composition on to an outer surface of the base layer.Any suitable manner of applying the sealing layer to the base layer iswithin the scope of the present disclosure.

In various embodiments, the second slurry may be substantially free ofboron nitride. In this case, “substantially free” means less than 0.01percent by weight. For example, the second pre-slurry composition maycomprise any of the components of the pre-slurry compositions and/orglass compositions described in connection with the first pre-slurrycomposition and/or first phosphate glass composition, without theaddition of a boron nitride additive. In various embodiments, the secondpre-slurry mixture may comprise the same pre-slurry composition and/orphosphate glass composition used to prepare the first pre-slurrycomposition and/or the first phosphate glass composition. In variousembodiments, the second pre-slurry composition may comprise a differentpre-slurry composition and/or phosphate glass composition than the firstpre-slurry composition and/or first phosphate glass composition.

In various embodiments, the first slurry and/or the second slurry maycomprise an additional metal salt. The cation of the additional metalsalt may be multivalent. The metal may be an alkaline earth metal or atransition metal. In various embodiments, the metal may be an alkalimetal. The multivalent cation may be derived from a non-metallic elementsuch as boron. The term “metal” is used herein to include multivalentelements such as boron that are technically non-metallic. The metal ofthe additional metal salt may be an alkaline earth metal such ascalcium, magnesium, strontium, barium, or a mixture of two or morethereof. The metal for the additional metal salt may be iron, manganese,tin, zinc, or a mixture of two or more thereof. The anion for theadditional metal salt may be an inorganic anion such as a phosphate,halide, sulfate or nitrate, or an organic anion such as acetate. Invarious embodiments, the additional metal salt may be an alkaline earthmetal salt such as an alkaline earth metal phosphate. In variousembodiments, the additional metal salt may be a magnesium salt such asmagnesium phosphate. In various embodiments, the additional metal saltmay be an alkaline earth metal nitrate, an alkaline earth metal halide,an alkaline earth metal sulfate, an alkaline earth metal acetate, or amixture of two or more thereof. In various embodiments, the additionalmetal salt may be magnesium nitrate, magnesium halide, magnesiumsulfate, or a mixture of two or more thereof. In various embodiments,the additional metal salt may comprise: (i) magnesium phosphate; and(ii) a magnesium nitrate, magnesium halide, magnesium sulfate, or amixture of two or more thereof.

The additional metal salt may be selected with reference to itscompatibility with other ingredients in the first slurry and/or thesecond slurry. Compatibility may include metal phosphates that do notprecipitate, flocculate, agglomerate, react to form undesirable species,or settle out prior to application of the first slurry and/or the secondslurry to the carbon-carbon composite. The phosphates may be monobasic(H₂PO₄ ⁻), dibasic (HPO₄ ⁻²), or tribasic (PO₄ ⁻³). The phosphates maybe hydrated. Examples of alkaline earth metal phosphates that may beused include calcium hydrogen phosphate (calcium phosphate, dibasic),calcium phosphate tribasic octahydrate, magnesium hydrogen phosphate(magnesium phosphate, dibasic), magnesium phosphate tribasicoctahydrate, strontium hydrogen phosphate (strontium phosphate,dibasic), strontium phosphate tribasic octahydrate and barium phosphate.

In various embodiments, a similar compound(s) to the additional metalsalt may be used as the additional metal salt. Similar compounds includecompounds that yield a similar compound to the additional metal salt inresponse to an outside stimulus such as, temperature, hydration, ordehydration. For example, similar compounds to alkaline earth metalphosphates may include alkaline earth metal pyrophosphates,hypophosphates, hypophosphites and orthophosphites. Similar compoundsinclude magnesium and barium pyrophosphate, magnesium and bariumorthophosphate, magnesium and barium hypophosphate, magnesium and bariumhypophosphite, and magnesium and barium orthophosphite.

While not wishing to be bound by theory, it is believed that theaddition of multivalent cations, such as alkaline earth metals,transition metals and nonmetallic elements such as boron, to the firstslurry and/or the second slurry enhances the hydrolytic stability of themetal-phosphate network. In general, the hydrolytic stability of themetal-phosphate network increases as the metal content increases,however a change from one metallic element to another may influenceoxidation inhibition to a greater extent than a variation in themetal-phosphate ratio. The solubility of the phosphate compounds may beinfluenced by the nature of the cation associated with the phosphateanion. For example, phosphates incorporating monovalent cations such assodium orthophosphate or phosphoric acid (hydrogen cations) are verysoluble in water while (tri)barium orthophosphate is insoluble.Phosphoric acids can be condensed to form networks but such compoundstend to remain hydrolytically unstable. Generally, it is believed thatthe multivalent cations link phosphate anions creating a phosphatenetwork with reduced solubility. Another factor that may influencehydrolytic stability is the presence of —P—O—H groups in the condensedphosphate product formed from the first slurry and/or the second slurryduring thermal treatment. The first slurry and/or the second slurry maybe formulated to minimize concentration of these species and anysubsequent hydrolytic instability. Whereas increasing the metal contentmay enhance the hydrolytic stability of the first slurry and/or thesecond slurry, it may be desirable to strike a balance betweencomposition stability and effectiveness as an oxidation inhibitor.

In various embodiments, the additional metal salt may be present in thefirst slurry and/or the second slurry at a concentration in the rangefrom about 0.5 weight percent to about 30 weight percent, and in variousembodiments from about 0.5 weight percent to about 25 weight percent,and in various embodiments from about 5 weight percent to about 20weight percent. In various embodiments, a combination of two or moreadditional metal salts may be present at a concentration in the rangefrom about 10 weight percent to about 30 weight percent, and in variousembodiments from about 12 weight percent to about 20 weight percent.

Method 400 may further comprise a step 250 of heating the compositestructure to form a sealing layer, which may comprise phosphate glass,over the base layer. Similar to step 230, the composite structure may beheated at a temperature sufficient to adhere the sealing layer to thebase layer by, for example, drying or baking the carbon-carbon compositestructure at a temperature in the range from about 200° C. (392° F.) toabout 1000° C. (1832° F.). In various embodiments, the compositestructure is heated to a temperature in a range from about 600° C.(1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (392°F.) to about 900° C. (1652° F.), or further, between about 400° C. (752°F.) to about 850° C. (1562° F.), wherein in this context only, the term“about” means plus or minus 10° C. Further, step 250 may, for example,comprise heating the composite structure for a period between about 0.5hour and about 8 hours, where the term “about” in this context onlymeans plus or minus 0.25 hours.

In various embodiments, step 250 may comprise heating the compositestructure to a first, lower temperature (for example, about 30° C. (86°F.) to about 300° C. (572° F.)) followed by heating at a second, highertemperature (for example, about 300° C. (572° F.) to about 1000° C.(1832° F.)). Further, step 250 may be performed in an inert environment,such as under a blanket of inert or less reactive gas (e.g., nitrogen,argon, other noble gases, and the like).

TABLE 1 illustrates a variety of slurries comprising pre-slurrycompositions, including phosphate glass compositions, prepared inaccordance with various embodiments.

TABLE 1 Example A B C h-Boron nitride powder 8.25 8.25 — Graphenenanoplatelets 0.150 0.150 — Surfynol 465 surfactant 0.2 0.2 — H₂O 60.060.0 52.4 Ammonium dihydrogen phosphate (ADHP) — —  11.33 Polyacrylicacid, sodium salt) — 0.50 — Cellulose — 5.0 — Glass frit 26.5 26.5 34.0Acid Aluminum Phosphate (Al:P = 1:2.5) 5.0 5.0 —

As illustrated in TABLE 1, two first slurries for oxidation protectionsystems, Slurries A and B, were prepared, such as first slurries appliedin step 220 of methods 200, 300, and 400, which may provide base layerson a composite structure. All amounts listed in TABLE 1 are listed ingrams. Slurry A comprises a pre-slurry composition, comprising phosphateglass composition glass frit and various additives including h-boronnitride, graphene nanoplatelets, a surfactant, and acid aluminumphosphate, in a carrier fluid (i.e., water). Slurry B comprises apre-slurry composition, comprising phosphate glass composition glassfrit and various additives including h-boron nitride, graphenenanoplatelets, a surfactant, a (sodium) salt of poly(acrylic acid) as asecondary flow modifier, and acid aluminum phosphate, and cellulose as aprimary flow modifier, in a carrier fluid (i.e., water). Slurry C may bea suitable second slurry which will serve as a sealing layer when heated(such as during step 250).

With combined reference to TABLE 1 and FIG. 3, the first slurries,Slurries A and B, were applied to 50 gram carbon-carbon compositestructure coupons and cured in an inert atmosphere under heat at 899° C.(1650° F.) to form base layers. After cooling, second slurry, Slurry C,was applied atop the cured base layers and the coupons were fired againin an inert atmosphere. After cooling, the coupons were subjected toisothermal oxidation testing a 760° C. (1400° F.) over a period of hourswhile monitoring mass loss. In FIG. 3, percent weight loss of thecomposite structure is depicted on the y-axis and heat exposure time760° C. (1400° F.) is depicted on the x-axis. The composite structurewith Slurry A applied to form the base layer is illustrated by data set305, while the composite structure with Slurry B applied to form thebase layer is illustrated by data set 310. As shown, after 24 hours ofheat exposure, FIG. 3 shows the composite structure represented by dataset 310 experienced less weight loss after 24 hours of heat exposurethan the composite structure represented by data set 305. Therefore, theoxidation protection system comprising Slurry B, which comprised aprimary flow modifier (cellulose) and a secondary flow modifier(poly(acrylic acid)), provides more effective oxidation protection thanthe oxidation protection system comprising Slurry A, which does notcomprise a primary or secondary flow modifier. As discussed above, theprimary flow modifier, cellulose, in Slurry B surprisingly interactsand/or bonds with the other compounds (and cellulose molecules) inSlurry B, facilitating more a uniform, continuous application of SlurryB and resulting base layer after heating (such as heating step 230 inmethods 200, 300, and/or 400), than Slurry A (without cellulose).Additionally, the secondary flow modifier of Slurry B, a salt ofpoly(acrylic acid) (e.g., sodium salt of poly(acrylic acid)), lowers theviscosity of Slurry B, such that Slurry B may be better distributed onthe composite structure via spraying. Therefore, the inclusion of aprimary flow modifier (e.g., cellulose) and a secondary flow modifier(e.g., poly(acrylic acid, sodium salt)) in a first slurry (e.g., SlurryB) provides comparable or better oxidation protection than a firstslurry without such components (e.g., Slurry A).

Benefits and other advantages have been described herein with regard tospecific embodiments. Furthermore, the connecting lines shown in thevarious figures contained herein are intended to represent exemplaryfunctional relationships and/or physical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships or physical connections may be present in apractical system. However, the benefits, advantages, solutions toproblems, and any elements that may cause any benefit, advantage, orsolution to occur or become more pronounced are not to be construed ascritical, required, or essential features or elements of the disclosure.The scope of the disclosure is accordingly to be limited by nothingother than the appended claims, in which reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather “one or more.” Moreover, where a phrase similar to“at least one of A, B, or C” is used in the claims, it is intended thatthe phrase be interpreted to mean that A alone may be present in anembodiment, B alone may be present in an embodiment, C alone may bepresent in an embodiment, or that any combination of the elements A, Band C may be present in a single embodiment; for example, A and B, A andC, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detaileddescription herein, references to “one embodiment,” “an embodiment,” “anexample embodiment,” etc., indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed. After reading the description, it will be apparent to oneskilled in the relevant art(s) how to implement the disclosure inalternative embodiments.

Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element herein is to be construed under theprovisions of 35 U.S.C. 112(f), unless the element is expressly recitedusing the phrase “means for.” As used herein, the terms “comprises,”“comprising,” or any other variation thereof, are intended to cover anon-exclusive inclusion, such that a process, method, article, orapparatus that comprises a list of elements does not include only thoseelements but may include other elements not expressly listed or inherentto such process, method, article, or apparatus.

What is claimed is:
 1. A slurry for an oxidation protection system on acomposite structure, comprising: a carrier fluid; a first pre-slurrycomposition comprising a first phosphate glass composition; a primaryflow modifier comprising at least one of cellulose or calcium silicate;and a secondary flow modifier comprising at least one of poly(vinylalcohol) or a salt of poly(acrylic acid).
 2. The slurry of claim 1,wherein the first pre-slurry composition comprises an acid aluminumphosphate, wherein a molar ratio of aluminum to phosphoric acid isbetween 1 to 2 and 1 to
 3. 3. The slurry of claim 1, wherein the firstphosphate glass composition is represented by the formulaa(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from:lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic,antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum,tin, titanium, chromium, manganese, iron, cobalt, nickel, copper,mercury, zinc, thulium, lead, zirconium, lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium,uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin,bismuth, cadmium, and mixtures thereof; a is a number in the range from1 to about 5; b is a number in the range from 0 to about 10; c is anumber in the range from 0 to about 30; x is a number in the range fromabout 0.050 to about 0.500; y₁ is a number in the range from about 0.100to about 0.950; y₂ is a number in the range from 0 to about 0.20; and zis a number in the range from about 0.01 to about 0.5;(x+y ₁ +y ₂ +z)=1; andx<(y ₁ +y ₂).